1. FIELD OF THE INVENTION
The present invention relates to power assisting assemblies for motor vehicles and particularly to heavy trucks designed for commercial use and often including a large number of forward gears/speeds.
2. PRIOR ART
The power transmission systems of many motor vehicles, particularly large trucks, are equipped with manual transmissions and friction-plate clutches. In manual operating gear-shifting mechanisms, the spring pressure of the clutch mechanism must be temporarily overcome to disengage the friction elements when the transmission is to be shifted from one gear to another. Typically, the clutch is operated by mechanical linkage connected to a foot pedal. The strong spring pressure of the clutch system that must be overcome for release renders clutches difficult to operate and has led to the development of a large variety of clutch boosters or similar assisting devices which are very often air pressure power assisting mechanisms.
Many power assist mechanisms employ systems that work off the movement of the pedal and result in a "soft pedal", i.e., one that requires little effort to operate. Unfortunately, these devices do not normally vary the travel of the pedal which may be up to ten inches. With any "high pedal" it is tempting for a driver to simply rest the left foot on the pedal between gear shifts. With a "soft pedal" the power assist can cause the driver to "ride the clutch" unwittingly which can in turn cause undue wear of the clutch assembly.
Some conventional power assist mechanisms also suffer an additional disadvantage in that they leave no provision for operation when air pressure is not available. In the present invention, the clutch mechanism is always mechanically operatively connected to the clutch pedal lever for "fail safe" operation.
These problems as discussed above should be solved in conjunction with the desired arrangement of providing both a long pedal stroke with a high mechanical advantage, and a short pedal stroke with a lower mechanical advantage because such an arrangement reduces the amount of time the clutch will be in slippage. This extends the life of the clutch.
The best possible solution would be a clutch mechanism that combines a long stroke high pedal mode with a higher mechanical advantage for precise control with a power-assisted low pedal mode and lower mechanical advantage which will operate the clutch quicker with less slippage and reduce the length of travel of the pedal thus requiring reduced effort on behalf of the driver. The low pedal mode will also provide for ease of operation during normal driving and will allow the driver to rest the heel on the floor instead of the pedal during driving which will also protect the clutch from partial release or damage. Ideally, the driver should be able to select the low pedal mode at will.
Finally, most clutch mechanisms employ a clutch brake. The clutch brake is used to stop the gear movement when in neutral during engine idle. The clutch brake is engaged by operating the clutch release system to its extreme position beyond that for disengaging the clutch. The most desirable clutch mechanism should eliminate the possibility of inadvertently engaging the clutch brake when shifting gears. The present invention allows the foot pedal in low pedal mode to travel all the way to the floor in low pedal mode without engaging the clutch brake. The linkage employed can only operate the clutch release system to its extreme position, thus engaging the clutch brake, in the high pedal mode The high pedal mode is usually used only when starting the truck and moving around a freight yard and the like where extremely precise control is desired and where the clutch brake is necessary. Of course, with experience, a driver may use the low pedal mode exclusively if so desired.
An example of the most pertinent prior art is applicant's invention disclosed in U.S. Pat. No. 3,677,379 where the pedal linkage is connected to a first lever, the clutch plate linkage is connected to a second lever, and the levers can be locked together or operated separately. When locked together, foot pressure on the pedal will be transmitted to the plate release mechanism.
The present invention has the functional equivalent of a structure that keeps the levers in operable mechanical connection at all times and thus provides a valuable "fail-safe" advantage. In addition to being always operable in manual, two power-assisted modes of operation are available: a high pedal mode and a low pedal mode. The two modes of operation employ a linkage that connects the pedal linkage to the clutch plate linkage and utilizes pin and shaft mounted levers instead of a more expensive rack and pinion mechanism used in the prior art. In addition, the present invention will be easier to maintain than prior art systems. Finally the linkage mechanism will prevent engagement of an associated clutch brake in the low pedal mode and provides the driver with a better "feel" of the system than other known devices.